Over the past half century, effective antibiotic treatment programs have made syphilis relatively uncommon in the United States, with less than 7,100 primary and secondary cases diagnosed in 2003. However, recent data indicates that reported cases are again increasing in subsets of the population, and periodic epidemics of syphilis have occurred for decades. In 1995, the number of new cases of syphilis worldwide was estimated to be 12 million per year. If untreated, syphilis can evolve from localized primary lesions (chancres) to disseminated, chronic infections, including secondary, latent, and tertiary forms.
As a syphilitic infection can produce a variable range of symptoms in humans, laboratory tests are often required to definitively diagnose an infection. Due to the inability to culture the causative organism, Treponema pallidum (T. pallidum) (TP), in vitro, a need exists for the development and optimization of in vitro methods for the detection of T. pallidum in diverse clinical specimens [Morse, Salud Publica Mex 5(Suppl 45):S698-S708, 2003]. While enzyme-linked immunosorbent assays (ELISAs) for Treponema are commercially available, they exhibit varying efficiencies at different disease stages [Schmidt et al., J Clin Microbiol 38:1279-1282 (2000)]. Several ELISAs based on whole cell lysate were developed which presented sensitivity of 93.3% to 100% and specificity of 95.5% to 99.8% [Castro et al., J Clin Microbiol 41:250-253 (2003)].
In recent years, several immunodominant and putatively pathogen-specific membrane lipoproteins of T. pallidum have been identified in patients with syphilis and in infants with congenital syphilis. These patients and infants developed antigen specific antibodies which could be detected by immunoblot and by enzyme immunoassay. Therefore, recent methods of detection use recombinant antigens, mainly the membrane-integrated proteins 47 kDa, 17 kDa and 15 kDa (TP47, TP17, and TP15, respectively), in treponemal ELISA tests. Although TP47 was the earliest identified, as well as the most abundant and highly immunogenic [Norgard et al., Infect Immun 54:500-506 (1986)], the later identified TP15 and TP17, present in lower amounts, are also strongly immunogenic [Purcell et al., Infect Immun 57:3708-3714 (1989); Akins et al., Infect Immun 61:1202-1210 (1993)].
Given the increase in reported cases of syphilis and the periodic epidemics, as well as the severity of the disease, a need continues to exist for sensitive and specific immunoassays for detection of Treponema pallidum. 